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<<What is homopolymer, and what is copolymer ?>>

Synthetic high polymers have structure which consists of repeated monomer units connected each other with chemical bonds, where a monomer means a unit component to form a high polymer. If a polymer consists of only one kind of monomers then it is called a homopolymer, while a polymer which consists of more than one kind of monomers is called a copolymer.
Although polyacetal resin is a polymer made of successive oxymethylene units (-CH2O-), second monomer units, i.e. comonomer units are added in the case of DURACON, therefore it is called a copolymer.


<<Acetal homopolymer vs. acetal copolymer>>

Both types of polyacetal resins are made of formaldehyde as major raw material. Then what is the difference between homopolymer and copolymer?

 

 

* Difference in thermal stability !

 

Although polyacetal resin is highly crystalline, tough, chemically resistant, and superior in friction and wear properties, it is decomposed or degraded under extremely high temperature or in the environment containing acidic components. Furthermore, this decomposition reaction continues successively, once it is started. It is sometimes called unzipping reaction, since it seems as if a zipper were unzipped one tooth by one tooth.

 

 

   ---------

   --> -------

   --> ------    (Decomposition)

   --> -----    (Decomposition is propagated to the next unit)

   --> -    (The propagation of decomposition
successively continues)

 

If the propagation of decomposition reaches the other end, this means that the entire polyacetal molecule has returned to original formaldehyde. When there are comonomer units in the molecule, however, the situation is as follows :

 

 

-------       (Decomposition)

-->------    (Decomposition is propagated to the next unit)

-->-----   (The propagation of decomposition stops at
the first comonomer unit encountered)

 

Since the chemical bonds contained in comonomer units are different in nature from those between oxymethylene units, the decomposition is stopped at this point. The thermal stability of DURACON is improved compared with homopolymer by virtue of the introduced comonomers.

 

 

 

* Some difference in physicochemical properties


By the introduction of comomer components, various properties other than thermal stability also change.
An oxyethylene unit (-CH2 CH2O-) which contains one more carbon atom is generally used as a comonomer unit. Although it depends on the amount of comonomer units introduced, it can be said that the nature of acetal copolymer is similar to that of acetal homopolymer, but that of oxyethylene polymer is also added to some extent.
The standard grade of acetal copolymer is compared with that of homopolymer in the table shown below for your reference.

 


Homopolymer
Copolymer
              Specific gravity
1.42
1.41
              Melting point
178℃
167℃
Short term mechanical property
              Tensile strength
690kgf/cm2
610kgf/cm2
              Tensile elongation at break
40%
60%
              HDT at 18.5kgf/cm2 load
124℃
110℃
UL temperature index
              Without impact
90℃
100℃
              With impact
85℃
95℃
              Electrical
105℃
110℃
              Flammability
94HB
94HB

 

It seems a general trend that homopolymer is superior to copolymer in the area of short term mechanical properties. In the long term, however, it is known that copolymer shows the better retention of various properties than homopolymer. In the case of creep rupture, for example, the stress to cause creep rupture of DURACON M25-44 is lower than that of the homopolymer counterpart at 1hr., but that of the former is conversely higher than that of the latter at 1,000 hrs. as shown in the following figure.

 

 

It is not a proper way, however, to draw a conclusion with regard the comparison between homopolymer and copolymer only from the above-mentioned results, but rather it is required actually to comprehensively judge what material is most suitable for the use environment based on various short term and long term properties including thermal stability, before the final selection of grades and the type of polymers.

 

 

* Reference *

  The only difference between polyacetal resin which is the polymer of oxymethylene, and polyethylene oxide which is the polymer of oxyethylene lies in the difference of the number of carbon atoms in the monomers by only one, but their properties are completely different.
It can be said that this makes high-polymer chemistry interesting but also difficult.

 

 
Polyoxymethylene
Polyethylene oxide
Structural formula
(-CH2O-)n
(-CH2CH2O-)n
Melting point
About 180℃
66℃
Density
About 1.4
About 1.2
Solubility
Not soluble in most organic
solvents at room temperature
Soluble in water at
room temperature
Hardness
Hard
Soft

 

 

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